The present invention relates to an improvement of a solid state relay, more particularly, to a solid state relay using a light coupling between a light emitting element and a light receiving element.
As FIG. 1(a) shows, a conventional solid state relay generally comprising a photocoupler 1 for coupling between input and output terminals in an electrical isolation condition, a bridge circuit for rectifying by supplying the output of the photocoupler 1, and a trielectrode AC switch (TRIAC) 3 to which the ouput of the bridge circuit is supplied. For example, in FIG. 1(a), a light emitting diode 5 of the photocoupler 1 emits light by inputting input signals, so that a photothyristor 6, coupled optically to the light emitting diode 5, turns on. When the photothyristor 6 turns on, a current flows to the gate of the TRIAC 3, so that the TRIAC 3 turns on, and outputs signals in response to the input signals. As the photothyristor 6 is operated only with a half wave cylce, a rectifier stack 2 is added in the solid state relay. Further, a limit resistance R.sub.1 and a snubber circuit comprising a resistance R.sub.3 and a capacitor C for the prevention of a TRIAC malfunction are provided in the TRIAC 3.
In the above state relay, discrete components for constructing the above circuit are provided on a glass epoxy resin substrate 4 as shown in FIG. 1(b), and each of the elements of the discrete components is electrically connected to each other in accordance with the circuit of FIG. 1(a). The discrete components on the substrate 4 are then coated by the epoxy resin.
As described above, the conventional solid state relay requires many circuit components, so that the shape of it is large and a wide space for packaging it in any electric apparatus is required. Furthermore the assembly such as electrical connections, takes a long time. Accordingly, the reliability of the assembled circuit as a solid state relay is low.